PROJECT SUMMARY/ABSTRACT Cochlear implant (CI) users who are 65 years or older typically show poorer speech perception performance than younger adult CI users, especially in competing background noise. The underlying mechanisms remain unclear. Whereas declined cognitive function has been proposed as the major factor, deteriorations in the auditory system have also been suggested to be the primary reason. As a result, there is no robust indicator for optimizing programming settings or habilitation strategies for older implanted patients. This study is designed to better understand underlying neurophysiological mechanisms of speech perception deficits in older CI users. Older listeners are known to have temporal processing deficits, and temporal cues are particularly important for discriminating speech in CI users. Therefore, the proposed study will focus on comparing peripheral and central auditory neural encoding of, and perceptual sensitivity to, temporal envelope cues between younger and older adult CI users. Aim 1 will assess group differences in temporal responsiveness of the auditory nerve in younger (< 60 years) and older (between 65 and 85 years) adult CI users. The group comparison will be done for three temporal properties of the auditory nerve: peri-stimulus neural adaptation, adaptation recovery and the accuracy of amplitude modulation encoding. Aim 2 will assess group differences in neural representation at the auditory cortex (i.e. cortical encoding) of, and perceptual sensitivity to, temporal gaps and voice onset time, as well as their association with speech perception in younger and older adult CI users. Aim 3 will evaluate contributions of differences in AN temporal responsiveness and cortical encoding of temporal gaps and amplitude modulation to variations in perceptual envelope sensitivity in individual adult CI users. For experiments in Aims 2 and 3, psychophysical procedures and electrophysiological measures of peripherally and centrally generated evoked potentials will be combined and applied in concert in the same CI users. In addition, cognitive functions will be evaluated and used as one of the predictors for behavioral results. Results of this study have substantial significance from both a basic science and a clinical management point of view. For example, these results will provide information about the contributions of peripheral vs central auditory functions, as well as auditory vs cognitive factors to perceptual sensitivity to temporal gaps and amplitude modulations, which is crucial for understanding underlying mechanisms of these auditory processes. These results will also provide information about whether the accuracy of neural encoding and processing of electrical hearing declines in older CI users, as well as at which level of the auditory system this decline, if any, exists. This information could potentially lead to developing objective tools for early identification of older CI patients who may need additional rehabilitation strategies. Given the importance of temporal processing for speech perception and the rapidly increasing number of older CI patients, experiments proposed in this application are of high scientific and translational significance.